Ribosomes, the universal cell organelles facilitating protein biosynthesis, are nucleoprotein assemblies (mw 2.3 MD), composed of 57-73 different proteins and 3 RNA chains of about 4500 nucleotides, arranged in two subunits of unequal size. Crystals, diffracting best to around 3 , have been grown fron intact large and from small ribosomal subunits (from haloarcula marismortui, H50S, and Thermus thermophilus, T30S, respectively). The latter are of high inteest since the small subunits are known to be extremely unstable and flexible and until recently yielded crystals diffracting to low resolution (7.3 at best, useful to 10-12 ). Data were collected with Synchrotron raditation (at F1/CHESS) at cryogenic temperatures. The brightness necessary for the collection of the higher resolution X-ray diffraction data introduces significant decay even at cryo temperature. Nevertheless, owing to the reasonable isomorphism of the recently improved crystals of the small subunits, reliable phases (as well as an electron density map) have been extracted at medium (6.5 ) resolution, using the combination of relatively large clusters with smaller metal compounds. The crystals of the large subunits are more problematic. It was found that their substantial radiation sensitivity accompanied by a low level of isomorphism, changes in unit cell axis (introduced by irradiation) and their non-isotropic mosaicity become more pronounced and therefore less tolerable with the increase in resolution, hampering smoot MIR phasing. Some of these problems were partially ovecome by using a beam of a rather small cross-section and irradiating the crystals part by part. Consequently, in favorable cases, up to 20 degrees were collected from one crystal. To gain insight on the structure of these crystals A 12 resolution MIR electron density map of H50S was constructed, using Ta6Br14, W3O2((acetate)6H2O)3)CFSO3, K6H(PW12O40)nH2O and Cs7(P2W17O61Co(NC5H5))nH2O for soaking. This map may indicate the reasons for the problematic nature of the crystals and provide hints on ways for their improvement.